1. Field of the Invention
The present invention relates to a current detection circuit configured to detect current flowing through a current detection resistor based on a voltage difference between two ends of the current detection resistor.
2. Description of the Related Art
There is a conventional current detection circuit that includes a current detection resistor disposed in a current pathway and can detect current flowing through the current detection resistor based on a voltage difference between two ends of the current detection resistor. According to such a conventional circuit, if a resistance value of the current detection resistor is small, a voltage drop across the resistor becomes smaller. However, if the voltage drop across the current detection resistor is small, the current detection circuit may not be able to detect a voltage different between two ends of the current detection resistor. Especially, when the measured current is small, the current detection circuit may fail in detecting a small voltage difference.
As discussed in Japanese Patent Application Laid-Open No. 2001-108712, an operational amplifier can amplify a voltage difference between two ends of a current detection resistor and accordingly the current detection circuit can detect a relatively small voltage difference.
However, if the current detection circuit includes an operational amplifier, accuracy in current detection may deteriorate due to a noise component generated by resistors that determine an amplification rate of the operational amplifier. In general, the resistors generate thermal noise. The amount of generated thermal noise is equal to Vn2=4·k·T·B·R, where k represents the Boltzmann's constant, T represents the absolute temperature, B represents a frequency band width, and R represents a resistance value. Furthermore, an output of the operational amplifier includes an offset voltage and noise which may also deteriorate the accuracy in current detection.
If the current detection resistor has a larger resistance value to obtain a large voltage drop across the current detection resistor, i.e., if the accuracy in current detection is prioritized, loss in the circuit increases correspondingly. For example, a portable device may employ a current detection circuit to detect a battery current. In such a case, it is useful to use a smaller current detection resistor so that the capability of the battery can be maximized. Therefore, the current detection circuit for a portable device is required to limit the voltage drop across the current detection resistor and increase the accuracy in current detection.